Other posters are correct - you would have to prove that the people knew about the condition ahead of time. That is usually rather easy to do (find their vet or anyone they discussed the condition with) but is only part of the story.
To be actionable, you must prove that they intentionally mis-represented the condition. A horse could be broken down, sick and have 6 months to live... nothing wrong with selling or buying it. The problem comes in if the owner represents this as a "healthy" horse.
The next problems that must be overcome would be the owner's defense if the matter was pursued. People, even when they are 100% right, fail to consider what the _other_ party will state in their defense.
Failing to get a vet check would be used against you (or anyone). The defendant would state "Vet Checks are standard. If they were that concerned about this supposed condition, they would have had a vet check. It obviously wasn't a concern."
And of course, there is what you just wrote:
Originally Posted by appylover31803
which I thought was fair based on his condition and lack of training)
The owner would absolutely use this against you, claiming you knew about this obvious "condition" and didn't care. Hey, that's why the price was so low! If not for the condition, the price would have been much higher, as you clearly stated! You just realized that the "training" was over your head and want to go back on the deal.
The only possible way around this is a written contract which specifically states the horse is "sound" and "healthy." Possible because they would still use the above defenses. A contract would (probably) constitute a representation by the seller. From there, if you get a vet to testify "I told them the horse wasn't sound/healthy on this date, and here is my written report" you would likely have them on breach or contract/fraud.
Legal action always sound good, but it never as easy as you think. Unfortunately, it is sometimes the only recourse.